if you only shot 22 stuff i would guess the 5/16 would be best but if you get it to close it will still leave dents I would say.
but if you can get your hands on some AR500 plate it will last forever or even
AR400 plate
Also MGM is having a sale on some 22rimefire auto reset targetsMGM Targets | Specials
retiredcpo

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those are all good designs but if you cut the parts out of AR500 plate it will last for ever! esp if only shot with rimfires
I shoot AR500 3/8 plate with everthing from 22S through 300 win mags
from 50 yards to 1000 yards the stell does not even scratch
retired3

Kansas has it right. There is no need for heavy duty steel for .22 rimfire spinners. I have some mild steel silhouette spinners that have been tagged thousands of times with no damage at all. I have shot them from 25 yards out to 100.